Tantalum capacitors are representative electrolytic capacitors. Tantalum capacitors are manufactured using for instance the following method. First, a sintered body is formed out of tantalum powder, next, a dielectric layer consisting of Ta2O5 is formed by anodic oxidation upon this sintered body. An electrolyte layer and an electrode (cathode) are further formed on this dielectric layer.
Tantalum possesses excellent capacitor properties. However, its mineral ore deposits are relatively scarce, which makes tantalum expensive and subject to considerable price fluctuations. For these reasons, there are ongoing studies concerning the use of niobium (Nb), which is more abundant and therefore less expensive, as a constituent material in capacitor anodes.
Anodic oxidation is used for forming dielectric layers (Nb2O5) in the manufacture of niobium capacitors. However, Nb2O5 is less stable than Ta2O5 and is relatively more prone to releasing oxygen (O2−), whereby Nb2O5 is converted into NbO and/or NbO2. Also, the closer to the niobium sintered body, the stronger the tendency of released oxygen to migrate towards the niobium sintered body. Thus, dielectric layers of tantalum oxide and niobium oxide exhibit the differences illustrated in FIGS. 8A and 8B.
As shown in FIG. 8A, a dielectric layer DL of Ta2O5 formed on a tantalum sintered body has a substantially uniform oxygen concentration across the whole layer. On the other hand, as shown in FIG. 8B, the composition of a dielectric layer DL of Nb2O5 changes first to NbO2 and then to NbO in the direction towards the interface BS. In other words, the closer to the interface BS, the more the oxygen concentration drops.
In general, niobium oxide is an insulator (dielectric) when the number of oxygen atoms bonded to one niobium atom is large, and a conductor when that number is small. In the case of FIG. 8B, Nb2O5 is an insulator whereas NbO is a conductor (in a strict sense, therefore, the layer DL in the figure cannot entirely function as a dielectric layer).
Conventional niobium electrolyte capacitors have the following drawbacks. As mentioned above, niobium oxide is less stable than tantalum oxide. In conventional niobium electrolyte capacitors, therefore, heat during soldering and the influence of voltage applied during use result in a tendency toward oxygen concentration changes in the vicinity of the interface. As a result, the thickness of the conductive layer (NbO) varies, which entails that the thickness of the portion acting as a dielectric in the layer DL also changes. Specifically, conventional niobium electrolyte capacitors have a drawback in that thermal or electric influences cause a deviation from the default permittivity value, which precludes obtaining the desired capacitor characteristics (capacitance, leakage currents, etc.).